1. Field of the Invention
The present invention relates to a rotary electric machine such as a starter motor that has a laminated armature core fixed to a rotatable center shaft.
2. Description of Related Art
An example of a rotary electric machine having a laminated armature core fixed to a rotatable center shaft is shown in JP-A-8-214481 or JP-A-2002-199626. The armature core disclosed therein is formed by laminating core sheets, each having a center hole, and by forcibly inserting a center shaft into the center holes of the laminated core sheets.
The following problem is involved in the conventional structure of the armature core. That is, a large amount of force is required to forcibly insert the center shaft into the center hole because the center hole is a continuous round hole. It is possible to reduce the inserting force by enlarging the diameter of the center hole (i.e., by reducing an amount of the diameter to be enlarged by forcibly inserting the center shaft). However, if the diameter of the center hole is enlarged too much, the laminated core sheets would not be firmly connected to the center shaft. Alternatively, knurls may be formed on the center shaft to firmly connect the center shaft to the laminated core sheets. However in this case, it is necessary to precisely control the diameter of the center hole, resulting in an increase in the manufacturing costs.